Unprotected sexual intercourse accounts for the vast majority of new HIV infections worldwide. A disproportionate number of these infections occur in women due to a variety of physiological and behavioral factors. The severity of this problem begs for the development of safe and effective chemical barriers ("topical microbicides") women can use to prevent mucosal HIV infection. Important insights for the development of topical microbicides can be derived from biological determinants for sexual HIV transmission. Three lines of evidence point to the HIV coreceptor, CCR5, is one of the most important. First, CCR5 is well represented on the mucosal epithelial and is the major coreceptor used to enter resident cells. In accordance, the majority of sexually transmitted HIV strains use this coreceptor for infection. Second, the genetic loss of CCR5 is correlated with strong natural resistance to infection. Notably, there is no apparent health consequence associated with the loss of CR5. Third, the natural capacity of certain individuals to produce high levels of HIV-suppressive CCR5. Third, the natural capacity of certain individuals to produce high levels CCR5 ligands correlates with uninfected status despite repeated exposure to HIV. In accordance, primate vaccine studies have correlated increased chemokine production with protection from virus challenge. Based on these findings, our central hypothesis is that effective vaginal microbicides can be based on biological molecules that block the CCR5 entry coreceptor for HIV. To evaluate our hypothesis, we will attempt to block vaginal R5 SHIV infection of rhesus macaques with microbicide formulations consisting of a CCR5 ligand/HIV inhibitor suspended in a carrier vehicle. We will examine two candidate microbicides, each with a distinct set of potentially beneficial features, first for in vitro anti-viral activity and for safety in animal models. Once characterized, they will then be tested for efficacy in the macaque infection model. One formulation will incorporate the -2 isoform of RANTES, a natural and selective CCR5 ligand with potent HIV- suppressive activity, and the second another natural CCR5 ligand, the HIVgp120-CD4 receptor complex formed during HIV attachment. The antiviral component of this formulation will be an immunoadhesin based on our single chain gp120-CD4 chimeric polypeptide, which we recently provided to be a specific inhibitor of R5 HIV infection. Each formulation will use a type of non-phospholipid liposome (Novasomes) that has appeared to be safe for vaginal application in our preliminary studies. This work will be accomplished within the context of three straightforward and synergistic projects. Upon completing this program we expect to have produced at least one candidate vaginal microbicide capable of preventing vaginal infection by a CCR5-tropic SHIV that will have practical utility in humans.